FIG. 2 shows an overview of a high photoelectric conversion efficiency solar cell using a single-crystal N-type silicon substrate, and FIG. 3 shows a schematic view of a cross-sectional structure of the same. A solar battery cell (which will be also simply referred to as a solar cell hereinafter) 200 has many electrodes having a width of 100 to tens of μm called finger electrodes (which will be also simply referred to as fingers hereinafter) 121 and 322 as collecting electrodes on a light receiving surface of an N-type substrate 110. As an interval of the finger electrodes adjacent to each other is generally approximately one to three mm. Further, two to four bus bar electrodes (which will be also simply referred to as bus bars hereinafter) 231 are provided as collecting electrodes to couple the solar battery cells. As methods for forming these electrodes, there are a vapor deposition method, a sputtering method, and the like, but a method for printing a metal paste having metal fine particles of Ag or the like mixed in an organic binder with the use of a screen plate or the like and performing a heat treatment at hundreds of degrees to bond the metal paste to a substrate is extensively used in terms of cost. Portions other than the electrodes are covered with an antireflection film 345 which is a silicon nitride film or the like. A P-type layer 312 which is opposite to a conductivity type of the substrate is formed on a front surface of the substrate. Finger electrodes 323 are also formed on a back surface side, and portions other than the electrodes are covered with a film 344 of silicon nitride or the like. A high-concentration N-type layer 313 having the same conductivity type as that of the substrate is formed on the outermost surface layer on the back surface.
Further, as a solar cell structure having high photoelectric conversion efficiency, there is a backside contact solar cell. FIG. 4 shows an overview of a back surface of a backside contact solar cell 400. On a back surface of a substrate 110, emitter layers 312 and base layers 313 are alternately aligned, and finger electrodes (emitter electrodes 322, base electrodes 323) are provided along upper sides of the respective layers. Furthermore, bus bar electrodes (an emitter bus bar electrode 432, a base bus bar electrode 433) to further collect currents obtained from these electrodes are provided. It is often the case that the bus bar electrodes are orthogonal to the finger electrodes because of their functions. A width of the emitter layer 312 is several mm to hundreds of and a width of the base layer 313 is hundreds of μm to tens of μm. Furthermore, an electrode width is generally approximately hundreds to tens of μm. FIG. 5 shows a schematic view of a cross-sectional structure of the backside contact solar cell 400. The emitter layers 312 and the base layers 313 are formed in the vicinity of the outermost surface layer on the back surface of the substrate 110. A layer thickness of each of the emitter layers 312 and the base layers 313 is no more than approximately 1 μm. The finger electrodes 322 and 323 are provided on the respective layers, and a surface of a non-electrode region (a region where no electrode is formed) is covered with a dielectric film (a backside protective film 344) which is a silicon nitride film, a silicon oxide film, or the like. For the purpose of reducing a reflection loss, an antireflection film 345 is provided on a light receiving surface side of the solar cell 400. Since no electrode is present on the light receiving surface, an incident light enters the substrate without being blocked, and hence photoelectric conversion efficiency is higher than that in the structure shown in FIG. 3.
The solar cell is processed into a photovoltaic module. FIG. 10 shows an overview of an example of a photovoltaic module. Solar cells 1000 fabricated as described above are laid like tiles in a photovoltaic module 1060. In the photovoltaic module 1060, several to tens of solar cells 1000 which are adjacent to each other are electrically connected in series to constitute a series circuit called a string. FIG. 11 shows an overview of the string. FIG. 11 corresponds to a schematic view of a back surface side in a module which cannot be usually seen. Moreover, fingers or bus bars are not shown. To achieve the series connection, as shown in FIG. 11, each P bus bar (a bus bar electrode connected to a finger electrode joined to a P-type layer of a substrate) of the solar cell 1000 is connected to each N bus bar (a bus bar electrode connected to a finger electrode joined to an N-type layer of the substrate) of an adjacent solar cell 1000 through a tab lead wire 1161 or the like. FIG. 12 shows a cross-sectional schematic view of the photovoltaic module. As described above, the string is constituted by connecting the plurality of solar cells 1000 through the tab lead wires 1161 connected to bus bar electrodes 231, respectively. The string is usually sealed in by a translucent filler 1272 such as EVA (ethylene vinyl acetate), a non-light receiving surface side is covered with a weatherable resin film 1273 such as PET (polyethylene terephthalate), and a light receiving surface is covered with a translucent light receiving surface protective material 1271 with high mechanical strength such as soda-lime glass.